Heat exchange element with hydrophilic evaporator surface

ABSTRACT

The present invention provides a heat exchange element for particular use in a film heat exchanger, such as a distillation system. The heat exchange element includes a substrate having a first surface and an opposing second surface. The first surface acts as a condenser surface. The heat exchange element further includes a composition disposed on at least a portion of the second surface. The composition has an exposed hydrophilic surface so as to provide a hydrophilic evaporator surface due to the composition being hydrophilic in nature. In contrast, the condenser surface is typically hydrophobic in nature. The use of the hydrophilic composition eliminates the nucleation of vapor bubbles and prevents gas film (or gas blanket) formation on the evaporator surface. As a result, the thermal resistance through the heat exchange element is decreased and the efficiency of the heat exchange element is increased.

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/226,067, filed Aug. 17, 2000, which is herebyincorporated by reference.

FIELD OF THE INVENTION

[0002] This invention relates to a heat exchange element and morespecifically, relates to a heat exchange element including a condensersurface having hydrophobic properties and an evaporator surface havinghydrophilic properties.

BACKGROUND OF THE INVENTION

[0003] Heat exchange elements are used in heat exchangers in a varietyof settings to accomplish a number of operations. Generally, a heatexchanger is a device that transfers heat from one gas or fluid toanother or to the environment. Heat exchange elements are commonlyemployed in a distillation apparatus for the evaporation of a liquid andfor its subsequent condensation. In the distillation apparatus, the heatexchange element may be in the form of flat bag-like elements of a thinsheet material placed against each other or may have other suitableforms. See, for example, U.S. Pat. Nos. 5,340,443, 5,512,141, and5,770,020, all of which are hereby incorporated by reference. Onesuitable use for a distillation apparatus is the production of freshwater from sea water.

[0004] Conventional sea water distillation systems, especially vaporcompression systems, include an evaporator-condenser type assembly inwhich the latent heat of condensing steam is exchanged to sea water.This latent heat in turn causes the sea water to boil into a vapor. Forpurpose of illustration, a conventional heat exchange element for use inan evaporator-condenser assembly is shown in FIG. 1 and is generallyindicated at 10. The heat exchange element 10 includes a thermalconductor 12 which is impermeable to both water and vapor. This thermalconductor 12 acts as a substrate for the element 10. The thermalconductor 12 has a first surface 14 on one side thereof and a secondsurface 16 on an opposite side thereof. The first surface 14 serves as acondenser surface where vapor is condensed into liquid and the secondsurface 16 serves as an evaporator surface where liquid is evaporated.In conventional heat exchange elements 10, both the condenser surface 14and the evaporator surface 16 are hydrophobic surfaces.

[0005] When the conventional heat exchange element 10 is used in adistillation system, the heat exchange element 10 is connected to bothan evaporator chamber (not shown) and a condenser chamber (not shown).Typically, the heat exchange element 10 is disposed between these twochambers. During operation of the distillation system, water droplets,generally indicated at 18, form on the condenser surface 16 as the vaporcondenses thereon to form a liquid condensate. A salt water film,generally indicated at 20, forms on the evaporator surface 16 as the seawater evaporates into a vapor.

[0006] One of the disadvantages associated with conventional heatexchange elements, such as the heat exchange element 10, is that theheat exchange element 10 is likely to have a number of heat transferregions present thereon during the heat transfer operation. Some ofthese regions include but are not limited to the following: convectiveheat transfer to liquid region, subcooled boiling, saturated nucleateboiling, forced convective heat transfer through liquid film, liquiddeficient region, and convective heat transfer to vapor. The flowpatterns of the fluid also vary depending upon the heat transfercharacteristics of the region in which the fluid is flowing. In theconventional heat exchange element 10, these various types of heattransfer regions are distributed over the heat surface in a stochasticmanners However, it will be understood that not all of these heattransfer regions may be present on the heat exchange element 10 at anyone time. The stochastic distribution of the regions results in theformation of bubble sites and dry spots that behave in a statisticalmanner in terms of boiling frequency and bubble size. The result is thatheat transfer at a macroscopic level is neither constant nor periodic.

[0007] For example, vapor bubbles often nucleate on the evaporatorsurface 16 resulting in pool or “pot” boiling. The pot boiling or bubblyflow, which occurs in the saturated nucleate boiling region, has a lowertotal heat transfer than other regions. Because heat transfer rates arelinked to efficiency rates, this lower heat transfer equates to lowerefficiency of the overall system. In addition and as illustrated in FIG.1, a layer of soft scales, generally indicated at 22, such as calciumcarbonate, frequently forms on the evaporator surface 16 because asuitable layer of liquid is not maintained on the evaporator surface 16.The formation of this layer 22 further reduces the heat transfereffectiveness in these regions of the evaporator surface 16 and thusreduces the effectiveness of the system. In order to prevent suchformations, the sea water in the evaporator (not shown) is generallyrecirculated in an attempt to make sure that a layer of liquid flowsalong the entire evaporator surface 16 to prevent drying out thereof;however, this requires a significant amount of energy. Furthermore, agas film or gas blanket, generally indicated at 24, may form on theevaporator surface 16 which increases the thermal resistance of thesystem. This further reduces the heat transfer efficiency of the heatexchange element.

[0008] Therefore, there is a continuing need for heat exchange elementswhich have improved heat transfer efficiency and require little, if any,outside energy to prevent soft scale formation.

SUMMARY OF THE INVENTION

[0009] The present invention provides a heat exchange element having awater and vapor impermeable substrate. The substrate has a first surfacewhich is typically hydrophobic and an opposing second surface. Acomposition is disposed on the second surface such that the compositionprovides an exposed hydrophilic surface. In one embodiment, thecomposition substantially covers the second surface so that the secondsurface essentially comprises a hydrophilic surface. According to thepresent invention, the first surface serves as a condenser surface ofthe substrate and the hydrophilic surface and any portion of the secondsurface not covered by the composition serve as an evaporator surface.One exemplary and preferred use for the heat exchange element of thepresent invention is in a distillation system for purifying sea water(or brine) to form fresh desalinated water.

[0010] Advantageously, the use of the hydrophilic composition to definethe evaporator surface eliminates the nucleation of vapor bubbles on theevaporator surface and also prevents gas film (or gas blanket) formationon this same surface. As a result, the thermal resistance through theheat exchange element is decreased and the efficiency of the heatexchange element is increased. Furthermore, since the evaporator surfaceattracts water, little, if any, power is required to recirculate the seawater or brine over the evaporator surface. The need to recirculate thesea water to prevent soft scale formation on the evaporator surface isreduced or eliminated, thereby, further increasing efficiency.

[0011] In another embodiment, a distillation system is disclosed inwhich the heat exchange element of the present invention is includedtherein. The distillation system has an evaporator chamber and acondenser chamber with the heat exchange element of the presentinvention being disposed therebetween. The evaporator surface of theheat exchange element is in communication with the evaporator chamberand the condenser surface is in communication with the condenserchamber.

[0012] These and other features and advantages of the present inventionwill be readily apparent from the following detailed description of theinvention taken in conjunction with the accompanying drawings, whereinlike reference characters represent like elements.

BRIEF DESCRIPTION OF THE FIGURES

[0013] Other objects, features, and advantages of the inventiondiscussed in the above summary of the invention will be more clearlyunderstood from the following detailed description of the preferredembodiments, which are illustrative only, when taken together with theaccompanying drawings in which:

[0014]FIG. 1 is a cross-sectional side elevational view of aconventional heat exchange element;

[0015]FIG. 2 is a cross-sectional side elevational view of a heatexchange element according to the present invention;

[0016]FIG. 3 is a perspective view of one exemplary heat exchangeelement; and

[0017]FIG. 4 is a cross-sectional side elevational view of the heatexchange element of FIG. 2 in a distillation system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Referring now to FIG. 2, a heat exchange element is providedaccording to the present invention and is generally indicated at 100.Heat exchange element 100 includes a substrate 102 which may be formedof any number of conventional materials and may have any number ofshapes depending upon the precise application. Although the substrate102 may be flexible in nature, it is preferred that the substrate 102 beformed so that it is rigid. Suitable substrates 102 also comprise thosesubstrates which are capable of being metallized. Exemplary materialsfor forming the substrate 102 include, but are not limited to, plastics,such as thermoplastics; metals, such as titanium; stainless steel; andcopper-nickel alloys; and any combination thereof. Preferredthermoplastic materials include, but are not limited to, polypropyleneand polyethylene. In addition, carbon loaded versions of thesethermoplastics are also suitable in the practice of the presentinvention.

[0019] The substrate 102 has a first surface 104 and an opposite secondsurface 106. The first surface 104 is intended to act as a condensersurface for producing a condensate. According to the present invention,a composition, generally indicated at 108, is disposed on at least aportion of the second surface 106. The composition 108 is selected so asto impart hydrophilic properties to at least the portion of the secondsurface 106 where the composition 108 is disposed. An evaporator surface109 is thus defined by both the hydrophilic composition 108 and thesecond surface 106. In the case where the composition 108 entirelycovers the second surface 106, the evaporator surface 109 is defined bythe composition 108. In the case where the composition 108 covers only aportion of the second surface 106, the evaporator surface 109 is definedby the composition 108 and that portion of the second surface 106 whichdoes not have the composition 108 disposed thereon. In the illustratedembodiment, the composition 108 covers the entire second surface 106.Thus, the entire evaporator surface 109 is a hydrophilic surface.

[0020] In one embodiment, the composition 108 is directly applied andbonded to the second surface 106 by using conventional processingtechniques, including depositing and bonding the composition 108 in aplasma environment or by chemical vapor deposition. See, for example,U.S. Pat. No. 5,763,063, which is hereby incorporated by reference. Itwill be appreciated that these processes are merely exemplary in natureand any number of other suitable processes may be used to deposit thecomposition 108 on the second surface 106. Alternatively, a primer layer(not shown) may be disposed between the composition 108 and the secondsurface 106 in order to promote improved bonding therebetween. Theprimer layer is selected so that sufficient and effective bondingresults between the primer layer and the second surface 106 and betweenthe primer layer and the composition 108.

[0021] The composition 108 of the present invention is formed of ahydrophilic material which is designed to withstand the operatingconditions of the element 100 and also provide the improved benefits ofthe present invention. Suitable hydrophilic materials include, but arenot limited to, metal oxides, such as titanium oxides (e.g., TiO_(x))and nickel oxides (e.g., NiO_(x)). Preferred metal oxides include, butare not limited to, higher order titanium oxides and nickel oxides.Metal oxides, and preferably higher order titanium and nickel oxides,are preferable since these materials maintain their hydrophilicproperties in the presence of boiling sea water and in reduced pressureenvironments. Higher order metal oxides are oxides of the metal withhigher oxygen content (i.e. more oxygen atoms than metal atoms permolecule) (e.g., HTiO₃ ⁻ or TiO₃.2H₂O). Metal oxides are generallyunaffected by anti-scaling and anti-foaming agents, which are commonlyemployed in sea water distillation systems. Other suitable hydrophilicmaterials include, but are not limited to, zeolites (including those atvarious silicon-aluminum ratios), aluminophosphates, polymer hydrogels(e.g. acrylate derivatives, such as Hypan® (available from LipoChemicals, Inc. of Patterson, N.J.) and diphenyl ethylene (DPE)).Generally, the hydrophilicity of the composition is sufficient to resultin 80% of the surface area of the evaporator portion of the heatexchanger to be wetted with a thickness of several molecular levels ofthe liquid to be evaporated.

[0022] According to one preferred embodiment, the composition is atitanium oxide or nickel oxide which is coated on the second surface106. Generally, the composition is coated on the second surface at athickness of from about 0.01 to about 2 μm and preferably at a thicknessof from about 0.1 to about 0.2 μm. According to another embodiment, thecomposition is coated on the second surface at a thickness of from about0.05 to about 0.4 μm.

[0023] During operation of the heat exchange element 100 in a suitableheat exchanger (not shown), water droplets 110 form on the first(condenser) surface 104 as the vapor condenses thereon to form theliquid condensate. In the particularly preferred application of the heatexchange element 100 in sea water evaporator-condenser systems, a saltwater film 112 forms on the evaporator surface 109 as the sea water isheated and evaporates therefrom. However, unlike the conventional heatexchange element 10 of FIG. 1, few, if any, scales form on theevaporator surface 109. Because the material required to form the scaledeposit remains in suspension with the sea water, scales are not formed.In addition, the lack of dry spots inhibits amalgamation of scales. Thisresults in greater heat transfer at the evaporator surface 109 and thusincreases the overall efficiency of the system in which the heatexchange element 100 is used.

[0024] Furthermore, another of the disadvantages associated with theconventional heat exchange element 10 (FIG. 1) is overcome by the designof the heat exchange element 100 of the present invention. Morespecifically, the gas film or blanket 24 (FIG. 1) which forms on theevaporator surface of the conventional heat exchange element 10 does notform on the evaporator surface 109 of the present invention. Because thegas film 24 is eliminated, greater heat transfer results at theevaporator surface 109 because the liquid to be evaporated may moreeasily come into contact with the evaporator surface 109. This alsoresults in an overall increase in the efficiency of the system.

[0025] While not being bound to any particular theory, it is believedthat the hydrophilic evaporator surface 109 offers benefits relative toconventional evaporator surfaces because the hydrophilic surface 109attracts the liquid to be evaporated and maintains a suitable layer ofliquid on the evaporator surface 109 during the evaporation process.During the evaporation process, a liquid film spreads over theevaporator surface 109. Because the liquid (e.g., sea water) isattracted to hydrophilic evaporator surface 109, the liquid spreads intoa fairly uniform film over the entire heat transfer surface (evaporatorsurface 109). During the evaporation process, the liquid to beevaporated is continuously fed to the evaporator surface 109 at apredetermined pressure and temperature which is sufficient to maintain adesired evaporation rate over the area of the evaporator surface 109. Byusing a hydrophilic material on the evaporator surface 109, the heattransfer surface is not marked by a variety of heat transfer regions butrather the heat transfer can be better controlled. The heat transfermechanism maintains “evaporation” as opposed to “boiling” on theevaporator surface 109. In other words, the system is forced into theforced convective heat transfer through liquid film region as opposed tothe saturated nucleate boiling region. Thus, the “boiling” action isprevented and the formation of the gas film or blanket is eliminatedbecause the liquid uniformly exists on the evaporator surface 109 due tothe hydrophilic attraction therebetween. The evaporator surface 109 doesnot dry out due to a uniform liquid layer being maintained thereon.

[0026] In the present invention, the liquid used in the evaporatorprocess is generally maintained in the two-phase forced convectionregion of heat transfer. The liquid is thus maintained below the pointof critical heat flux (CHF), i.e., the point of complete evaporation ofthe liquid film. When a liquid reaches CHF, dry spots form on theevaporator surface 109 because there is a lack of liquid in contact withthe evaporator surface 109 at these dry spot locations. This causes areduction in heat transfer and reduces the overall efficiency of thesystem. Avoiding the CHF condition places a limit on the level ofevaporation that may be achieved for a given area of the evaporator (notshown) and at a given heat flux. However, this limit places little, ifany, burden on many systems of industrial size and typical flow ratesfound therein.

[0027] It is desirable to maintain the liquid film in the two-phaseforced convection region heat transfer region because in this region,the liquid flows along the evaporator surface 109 while evaporationtakes place. Dry out conditions are avoided because the liquid ismaintained below CHF and at the same time the liquid effectivelyevaporates at the desired rate so as to produce efficient heat transfer.By staying in this region, the rate of heat transfer is optimized andthe need to recirculate liquid to prevent drying is eliminated. Also, bystaying in this region, the formation of entrained bubbles is alsoeliminated. The hydrophilic nature of the surface produces thefoundation of uniformly distributed water molecules which in turn formsa uniform film of water above that foundation. The thinner the layer,the less effective the heat transfer.

[0028] The heat exchange element 100 according to the present inventionmay have any number of configurations and shapes so long as at least aportion of the evaporator surface 109 includes the hydrophiliccomposition 108. For example, the heat exchange element 100 may have aflat sheet design, a bag design, or a plate design. One suitable designis set forth in International Publication No. WO 98/31529, which isherein incorporated by reference in its entirety. This applicationdiscloses bag-like elements which stretch during use to provide forbulging during the pressurization of the interior of the element and isgenerally illustrated in FIG. 3.

[0029]FIG. 3 shows an individual heat exchange element 150 for use in afilm heat exchanger (not shown). The heat exchange element 150 is formedof two oppositely positioned plastic heat exchange films 152 whichattach to one another at the top and the bottom of the element 150 andon the side which is on the left in the Figure. The films 152 areadditionally bonded to each other along mutually parallel obliquebonding lines 154, which divide the interior of the element 150 intoparallel ducts 156 extending from one side of the element to the other.The hot vapor to be condensed is introduced into the interior of theelement 150 from its at least partly open side 158, which is on theright of the Figure. The vapor flows in the direction of the arrows setforth in the Figure. The condensate formed from the vapor in the ducts156 leaves via the outlet opening 160 in the lower left comer of theelement 150. The liquid to be evaporated is directed between theelements 150 and is evenly distributed over exterior surfaces 153 of theheat exchange films 152. In accordance with the present invention, theexterior surfaces 153 of the heat exchange films 152 are thus providedwith a layer of hydrophilic material resulting in the exterior surfaces153 being hydrophilic. It will be appreciated that this illustratedembodiment is merely exemplary in nature and any number of heat exchangestructures may be used in the practice of the present invention so longas a portion of the evaporator surface 109 is made hydrophilic.

[0030] Now referring to FIG. 4 in which an exemplary distillation systememploying the heat transfer element 100 of the present invention isillustrated and generally indicated at 200. The distillation system 200includes a condenser chamber 202 and an evaporator chamber 204 with theheat exchange element 100 being disposed therebetween. The condenser andevaporator chambers 202, 204, respectively, are typically not in directcommunication with each other. The hydrophilic evaporator surface 109 ofthe heat exchange element 100 faces the evaporator chamber 204 and is incommunication therewith. Similarly, the condenser surface 104 faces thecondenser chamber 202 and is in communication therewith. Thedistillation system 200 operates like other traditional distillationsystems in that there is an evaporation process for evaporation of aliquid and a condensation process for the subsequent condensation ofvapor to a liquid condensate. This system 200 finds particular utilityin sea water distillation applications for producing purified water fromthe sea water.

[0031] According to one preferred embodiment, the heat exchange element100 is used in a vacuum vapor compression distillation process, both ofthe mechanical and thermal types. In thermal vapor recompressionevaporators, the inertia of the steam is used for recycling part of theevaporators through ejectors. In mechanical vapor recompressionevaporators, all of the vapor is recycled back as heating steam usingvapor compression with high pressure fans or compressors.

[0032] While the invention has been particularly shown and describedwith reference to the preferred embodiments thereof, it will beunderstood by those skilled in the art that various changes in form anddetail may be made therein without departing from the spirit and scopeof the invention.

What is claimed is:
 1. A heat exchange element comprising: a substratehaving a first surface and an opposing second surface, the first surfaceacting as a condenser surface; and a composition disposed on at least aportion of the second surface, the composition having an exposedhydrophilic surface so as to provide a hydrophilic evaporator surface,wherein the substrate is formed of plastic.
 2. The heat exchange elementof claim 1, wherein the composition substantially covers the secondsurface.
 3. The heat exchange element of claim 1, wherein the firstsurface comprises a hydrophobic surface.
 4. The heat exchange element ofclaim 1, wherein the substrate is formed of a water and vaporimpermeable material.
 5. The heat exchange element of claim 1, whereinthe substrate is formed of a plastic material selected from the groupconsisting of polypropylene and polyethylene.
 6. The heat exchangeelement of claim 1, wherein the composition comprises a metal oxide. 7.The heat exchange element of claim 1, wherein the composition comprisesat least one of titanium oxide and nickel oxide.
 8. The heat exchangeelement of claim 1, wherein the second surface has hydrophilicproperties such that liquid flowing therealong during an evaporationprocess is maintained below the critical heat flux value for the liquid.9. A distillation apparatus comprising: an evaporator having anevaporator chamber; a condenser having a condenser chamber; and a heatexchange element disposed between the evaporator and the condenser suchthat the heat exchange element is in fluid communication with thecondenser and evaporator chambers, the heat exchange element comprising:a substrate having a first surface and an opposing second surface, thefirst surface acting as a condenser surface; and a composition disposedon at least a portion of the second surface, the composition having anexposed hydrophilic surface so as to provide a hydrophilic evaporatorsurface, wherein the substrate is formed of plastic.
 10. Thedistillation apparatus of claim 9, wherein the composition substantiallycovers the second surface.
 11. The distillation apparatus of claim 9,wherein the first surface comprises a hydrophobic surface.
 12. Thedistillation apparatus of claim 9, wherein the substrate is formed of awater and vapor impermeable material.
 13. The distillation apparatus ofclaim 9, wherein the composition comprises a metal oxide.
 14. Thedistillation apparatus of claim 9, wherein the composition comprises atleast one of titanium oxide and nickel oxide.
 15. The distillationapparatus of claim 9, wherein the apparatus is used to distill sea waterinto desalinated water.